The present invention relates to a chisel for an impact tool and a method of manufacturing a chisel for an impact tool.
There is known in the prior art a type of impact tool used to crush or break objects such as rock beds and concrete. The objects are struck having a chisel mounted on one end of a cylinder with a piston reciprocating in the cylinder. One problem associated with this type of impact tool is that the chisel tends to wear quickly at its tip due to being repeatedly struck against the objects to be crushed.
In order to improve the durability of such a chisel, Examined Japanese Utility Model Publication 59-90581 proposed a core member, made of a material harder than the chisel, press-fit into an axial hole formed in the chisel and opening at the tip of the chisel.
In such a chisel, impacts are applied first to the chisel body and then transmitted to the core member. Thus, it is essential that the rear end face of the core member be in tight contact with the closed end of the axial hole formed in the chisel body so that the two members will act as one unit. Otherwise, stresses resulting from impact forces would concentrate on part of the non-contact portion, causing hardening of the contact and non-contact surfaces. Such hardened surfaces tend to sustain damage due to metal fatigue.
In order to solve this problem, Examined Japanese Patent Publication 3-73430 proposes to interpose a packing made of a soft metal such as lead between the closed end of the axial hole in the chisel and the rear end face of the core member.
This improved chisel has, however, a problem in that, due to unavoidable dimensional errors for the hole in the chisel, the core member and the soft metal packing, it is very difficult to bring the rear end face of the core member into tight contact with the closed end of the hole. Rather, in the initial stage of use of the chisel, the core member is likely to be in partial contact with the chisel body, so that the core member and the chisel body tend to be easily damaged.
Also, when press-fitting the core member into the hole in the chisel with the soft metal packing placed in the hole, the packing tends to displace to one side of the core member. This causes the rear end face of the core member to be brought into partial contact with the closed end face of the hole. Thus, impacts applied to the core member tend to be concentrated on the partial contact portion. The core member is thus subjected to unduly strong local forces, so that it can be easily damaged.
Moreover, in order to bring the rear end face of the core member into sufficiently tight and uniform contact with the closed end face of the hole formed in the chisel body, both of them have to be formed semispherically. Formation of such semispherical surfaces requires long periods of time and high manufacturing costs.
An object of the invention is to provide a method of manufacturing a chisel which makes it possible to bring the rear end face of the core member into tight contact with the closed end face of the hole formed in the chisel body once the core member is inserted into the hole.